In minecraft when you just begin you have to get lots of things down before night and hope you dont die.so when you first spawn your in "Dark era" (Dark ages is copyrighted).In "Dark era" you are in easy mode until you either smelt your first iron or survive 10 days.if you survive 10 days you get "survival expert" achievment.after you get one of those achievments you go into "Survival era".you are in normal normal mode until you either mine diamonds or get 10 achievments.after you get out of that era you go into "Adventure era".you are now in hard and have LOTS of new things you need to do.complete the game explore a stronghold complete the 3 new quests and survive.
note italic words mean new additions.